1. Field of the Invention
This invention relates to a speed control apparatus for a motor which is particularly suitable for a motor driving a load which varies at a predetermined period. The invention also pertains to a speed control method for such a motor.
Since a brushless DC motor is typically used for driving loads which tend to vary, this invention specifically relates to a method of controlling speeds of the brushless DC motor by measuring the revolution period from a position detection signal and computing the revolution number.
Typically, this invention is suitably applied to a motor for driving a compressor.
Recently, in order to change cooling or heating capability of room air conditioners, a control apparatus has typically been available which controls the revolution number of a motor for driving a compressor by using an inverter.
The revolution number of this type of motor can be controlled within a range from about 2,000 rpm to about 6,000 rpm.
It is expected that if the controllable range of the revolution number is further extended, then control performance for conditioning the room air increases over a wider range so that heating capability can be enhanced under high revolutions and power consumption and noise can advantageously be reduced under low revolutions.
2. Description of the Related Art
Conventionally, however, running of the motor at a low revolution of about 1,000 rpm difficult to practice because vibration and noise increase for the reasons described below.
More particularly, a compressor used for room air conditioner or a refrigerator is typically housed hermetically together with the drive motor in a chamber and irrespective of the type of the compressor, rotary or reciprocation type, load torque applied to the compressor motor greatly pulsates in relation to revolving positions, attended with maximum loading torque amounting up to about thrice average loading torque, and the pulsation repeats at a period of one revolution.
Reference should then be made to FIG. 15 which graphically illustrates changes in load torque T.sub.L, output torque T.sub.M of the motor and revolution number N in relation to revolving angles of the motor.
In a revolving angle region designated by B where the load torque T.sub.L exceeds the output torque T.sub.M, an angular acceleration due to a decrease in the revolution number N takes place with the result that revolving inertial torque represented by a product of the angular acceleration and a moment of inertia J owned by a rotary axis system of the motor occurs which cooperates with the output torque T.sub.M to balance with the load torque T.sub.L.
Conversely, in a revolving angle region designated by A where T.sub.L &lt;T.sub.M, revolving inertial torque due to an increase in the revolution number takes place. Thus, the revolving inertial torque corresponds to a torque difference T.sub.M -T.sub.L and as a result, balance in torque is maintained between the output motor and the loading compressor.
Consequently, in the motor driven compressor, the revolving pulsation occurs during one revolution, causing vibration and noise throughout the compressor chamber.
Especially, when running speed of the motor extends to low revolutions, a decreased revolution number causes the amplitude of the revolving pulsation to increase and the frequency thereof to decrease if an angular acceleration equivalent to that under high revolutions occurs.
This leads to occurrence of a vibration whose amplitude increases with the revolving pulsation and frequency decreases therewith.
Conventionally, the extension of the range of revolution number of the compressor motor to low revolutions has therefore required bulky vibration and noise preventive devices against the magnified vibration and noise and has been difficult to practice.
A torque control apparatus developed by the present inventors to solve the above problems is seen in, for example, Japanese Pat. Application No. 123639/84, according to which a pattern of load torque is stored in advance and the stored torque pattern data is read at the rate of a predetermined revolving angle to control output torque.
This apparatus is however based on a premise that the load torque pattern related to revolving angles is known and is therefore sufficient as far as this premise is met, but it is still unsatisfactory facing problems of inflexibility of patterning to a desired load torque pattern and necessity of detection of a reference position relative to the revolving angles, thus leaving behind tasks to be studied.
To control speeds of the brushless motor, a method has been proposed as disclosed in Japanese Pat. Unexamined Publication No. 44991/84, which comprises five processings respectively directed to measurement of time for each 60.degree. electrical angle from a position detection signal, computation of a time for 60 n.sub.1 electrical angle where n.sub.1 is a positive integer, computation of a revolution number from the computed time, computation of proportional, integration and differential terms of a difference revolution number between the computed and so detected revolution number and a command revolution number, and determination of an output voltage of an inverter based on the difference revolution number. However, the measurement of the time for 60.degree. electrical angle is asynchronous with the ensuring processings. To be specific, the determination of the inverter output voltage following the computation of the time for 60.degree. electrical angle is based on the computed time and is retarded from the computing processing, thus leading to a problem that response speed of a speed control system is decreased.
Especially, problems encountered in using the brushless DC motor, controlled by the conventional speed control method, as the compressor motor are as follows. More particularly, as described previously, the compressor used for a room air conditioner or a refrigerator is typically housed hermetically together with the drive motor in the chamber and in any of rotary type compressor and reciprocation type compressor, the load torque applied to the compressor motor greatly pulsates in relation to revolving positions, so that the maximum load torque amounts up to about thrice the average load torque. And, the pulsating load has a pattern almost determined relative to the revolving angles. This accounts for the fact that as far as speed controlling is effected asynchronously with the position detection signal synchronous with the revolving angles as in the conventional speed controlling, it is difficult to determine the inverter output voltage in quick response to the load changeable with the revolving angles and consequently, the difference between the motor output torque and load torque is increased to generate the revolving pulsation whose amplitude is increased and frequency is decreased under low revolutions, causing magnified vibrations throughout the chamber.